As is known in the art, multipath interference may occur when transmitting a signal from a transmitting antenna to a receiving antenna via a communication channel. More specifically, even if the transmitting antenna directly transmits a main component of the signal to the receiving antenna, other components of the transmitted signal may reflect off of buildings, moving or stationary objects, or the terrain, collectively referred to as “multipath sources,” before arriving at the receiving antenna. These multipath components may interfere with the main signal component at the receiving antenna, and may be delayed in time relative to the main signal component as well as to each other, and may have weights that differ from one another, and thus may make it difficult to demodulate the main signal component so as to obtain the information that it carries. Additionally, if the transmitting or receiving antenna is in motion, e.g., is in a vehicle or a communications satellite, then the multipath signal components may vary over time as the transmitting and receiving antennas move relative to one another and relative to the multipath sources. Moreover, such motion may Doppler-shift some of the multipath signal components, further increasing the difficulty of demodulating the main signal. A communications channel experiencing interference from multipath signal components may be more succinctly referred to as a “multipath channel.”
A variety of equalizers have been developed to attempt to address multipath signal components using adaptive filtering. One such technique, referred to in the art as an “adaptive zero-forcing equalizer receiver,” is schematically illustrated in FIG. 1. As illustrated in FIG. 1, the receiver 100 includes adaptive equalizer 110, receive antenna 120, signal digitizer and conditioner 130, and demodulator 140. Adaptive equalizer 110 includes fixed tapped delay line circuitry having a first plurality of spaced taps 111, a second plurality of spaced taps 112, multipliers (circles with an “X” inside), summing circuitry (circles with a sigma “Σ” inside), subtractor 113, and detector 114. The value yk denotes the equalizer internal signal value at time instant k, the value k denotes the equalizer internal error signal value at time instant k, and ak denotes the equalizer output signal at time instant k.
Receive antenna 120 is configured to receive a signal having multipath signal components, e.g., from a transmitting antenna (not shown) via a multipath channel. Signal digitizer and conditioner obtains and digitizes the signal and multipath signal components received by the antenna, and also may filter or otherwise condition the signal and multipath signal components. The digitized signal, which varies as a discrete function of time t, then is provided as input “Input(t)” to adaptive equalizer 110. Taps 111, 112 of adaptive equalizer 110 are divided over the anticipated time delay spread of the multipath signal components, and each operates on a single signal segment that is separated from other signal segments by the time delay value T. During operation, the equalizer illustrated in FIG. 1 may adaptively vary the relative weights of the different time signal segments over time so as to force the error signal generated by subtractor 113 to zero, and thus so as to align the main signal contribution with the multipath signal components in a coherent manner, and reduce the amount of multipath interference in the output signal provided to demodulator 140. More specifically, the second plurality of spaced taps 112 are used together with error signal k to provide feedback for a relative weight adjustment applied to the output of the first plurality of spaced taps 111. For additional information about zero-forcing equalizers, see Proakis, “Contemporary Communication Systems Using MATLAB,” 2nd Edition, ISBN 0-534-40617-3, the entire contents of which are incorporated by reference herein. In addition to the zero-forcing equalizer example described, adaptive weighing of the time delay taps may be derived using a minimum mean-square error (MSE) criteria such as known in the art.
Note that at any given moment in FIG. 1, some of the multipath signal components may be relatively close in time to one of the tap delays, while others may be relatively far in time from any of the tap delays. If a given multipath signal component delay is relatively close to a tap delay, then that component may be satisfactorily equalized. However, if the multipath signal component delay is relatively far in time from any of the tap delays, then that component may be insufficiently equalized, thus degrading demodulation of the signal. As is known in the art, increasing the number of taps may facilitate equalization over a greater bandwidth. Increasing the equalizer bandwidth by 2 requires decreasing the time duration for each tap by a factor of 2 which doubles the total number of taps. As such, the complexity of the adaptive equalizer receiver may increase significantly based on increases in the desired equalization bandwidth. Additionally, for circumstances where the transmitting and receiving antennas and multipath sources are moving relative to one another, the delays of each of the multipath signal components also may move relative to the tap delays. Accordingly, the performance of adaptive equalizer receiver 100 may be limited by the ability of multipliers, summing circuitry, and any algorithms operating therein, to keep up with dynamic changes in the time delayed signal components generated by the multipath channel.
The adaptive equalizer receiver is a technique that may be generally applied to received waveforms with multipath. For CDMA received waveforms, RAKE receivers are typically used. For additional information about adaptive equalizer receivers, as well as RAKE and other previously known techniques for addressing multipath signal components, see Calhoun, “Third Generation Wireless Systems, Volume 1, Post-Shannon Signal Architectures,” Artech House, Boston, pages 344-376 (2003), the entire contents of which are incorporated by reference herein.
Other approaches to addressing multipath interference may rely on spatial diversity, such as multiple-input and multiple-output (MIMO) techniques, in which both the transmit side and the receive side use multiple antennas. However, MIMO implementations may have limited application because multiple transmitters and receivers with sufficient spatial diversity may be incompatible with some practical link geometries. Other techniques, such as used with digital video broadcasting (DVB) standards may use training or pilot signals into the block coding so as to facilitate synchronization and to reduce the effects of multipath interference. Other approaches may utilize blind implementation techniques based on maintaining the known spectral characteristics of the transmitted signal. However, such techniques may rely upon the receive side having a relatively large amount of a priori knowledge about the transmitted signal, thus limiting flexibility in the transmitted signals.
Thus, what is needed is an improved technique for reducing the effects of multipath signal components.